Electrochemical capacitors, such as electric double layer capacitors, redox capacitors and hybrid capacitors, are long-life compared with secondary batteries, and have superior high cycle characteristics and instantaneous charge/discharge characteristics, consequently demand for electric double layer capacitors has rapidly been increasing in recent years for memory backup power supplies. This type of electrochemical capacitor is also attracting attention for uses such as auxiliary power supplies in vehicle batteries and motors, and development of high capacity electrochemical capacitors is under way.
In one example of this type of electrochemical capacitor, separators are interposed between polarizable electrodes formed on a pair of collectors, this construction being laminated or wound, impregnated with an electrolyte solution and housed in a case. The electrodes are formed from thin sheets of material obtained by mixing a carbon powder such as active carbon with a binder such as polytetrafluoroethylene.
It is known that, since an electric double layer is produced at the interface between the electrolyte solution and conductive material, the capacity of the capacitor becomes larger, the larger the surface area of the conductive material is. For this reason, the pores in the conductive material are made smaller to increase the surface area of the conductive material, and the capacitance of the capacitor increases, the larger the surface area is. However, there is a disadvantage that if the pores are made small, the mobility of the electrolyte solution decreases, and the internal resistance of the capacitor also increases. It is thus very difficult to manufacture an electric double layer capacitor which not only has low internal resistance, but also has large capacity.
To increase the capacity of electric double layer capacitors, or from the viewpoint of reducing internal resistance, various improvements in conductive materials have been reported. For example, a redox type capacitor wherein the conductive material itself stores electricity by an oxidation-reduction reaction, and a hybrid capacitor which uses a Faraday reaction at one electrode, have been proposed (Nonpatent document 1).
[Nonpatent document 1] “High capacity capacitor techniques and materials”, II—“Latest trends in electric double layer capacitors and super capacitors”, CMC Co., Ltd., January 2003”
On the other hand, the binder which is the other component of the electrode must be a material which has resistance to electrolyte solutions, electrochemical stability and heat resistance, and resins such as polytetrafluoroethylene and polyvinylidene fluoride are specifically used for this purpose.
If the amount of binder used is increased, there is the disadvantage that the internal resistance of the electrode increases, so in order to reduce internal resistance, it is desirable to reduce the amount of binder used. On the other hand, if the amount of binder used is reduced, adhesion between the electrode and the collector becomes poorer, which again leads to an increase of internal resistance.
Recently, a polyimide silicone resin composition has been proposed as a binder composition offering good adhesion between electrodes and collectors, and has superior heat-resistance and chemical resistance (Patent document 1). However, when this composition was used as a binder, although it had chemical resistance, it swelled up when immersed in an electrolyte solution.
[Patent document 1] Japanese Unexamined Patent Application Laid-Open (JP-A) No. 2002-289196
A polyimide resin and polyamidoimide resin have also been proposed as the binder for the adhesive layer which sticks the electrode and collector together (Patent document 2). However, although these resins have high heat resistance and chemical resistance, they have poor adhesion to metals and organic resins such as polytetrafluoroethylene.
[Patent document 2] JP-A No. 2004-48055